In an image forming apparatus of an electrophotographic process, for forming an electrostatic latent image on a charged electrophotographic photoreceptor, there is known a method of scanning the electrophotographic photoreceptor with plural light beams (hereinafter referred to as “multi-beam method”). The image forming apparatus of such multi-beam method is considered effective for achieving a higher speed in the image forming process, because of following advantages 1) to 3).
1) An image forming apparatus employing n laser beams (n being a natural number), with a scanning speed of the laser beam and a print speed selected same as those in the case of employing a single laser beam, can increase a density of scanning lines to n times, thereby achieving an image recording of a high resolution; 2) In the case of selecting the scanning speed of the laser beam and the density of the scanning lines same as those in the case of employing a single laser beam, the print speed can be increased to n times; 3) In the case of selecting the print speed and the scanning density of the laser beam same as those in the case of employing a single laser beam, it is possible to reduce the scanning speed of each laser beam (namely reducing, to 1/n times, a revolution of a rotary polygon mirror which reflects the laser beam to irradiate the electrophotographic photoreceptor thereby forming the electrostatic latent image thereon) whereby a mechanism for rotating the rotary polygon mirror can be simplified to achieve a cost reduction.
For the image forming apparatus of such multi-beam method, there is proposed an image forming apparatus in which plural laser beams are respectively deflected to simultaneously scan a scanned member such as an electrophotographic photoreceptor and an image is formed by scanning with plural scan lines in a single main scanning operation and which employs a surface emitting laser capable of easy array formation (VCSEL: vertical cavity surface emitting laser) as a light source of the exposure device and increases the number of simultaneously scanning laser beams (namely the number of scanning lines simultaneously scanned by laser beams) thereby achieving a higher quality of the image and a higher speed in image formation (see, for example, patent document 1).
On the other hand, in the image forming apparatus, there are being required not only a higher quality of the image and a higher speed of image formation, but also a downsized configuration and a longer service life for providing high-quality images over a prolonged period in stable manner. The service life of the image forming apparatus often depends on a service life of a photoreceptor employed therein, and such service life is known to result from a gradual deterioration of the image forming characteristics of the photoreceptor by mechanical and chemical actions in the course of repetition of charging, exposure, development, transfer and cleaning steps in the electrophotographic process.
It has been already known that the above-mentioned deterioration in the image quality by the chemical action is caused by the progress of oxidation of a binder resin and the progress of oxidation of a charge transport material in the photoreceptor by ozone generated in such repeated steps. It has also been known that the above-mentioned deterioration in the image quality by the mechanical action is caused by the progress of abrasion of the photoreceptor and/or the generation of scratches thereon, which are due to a deposit or the like generated in repeated steps of the electrophotographic process. Particularly, in the case where the photoreceptor is made smaller in diameter for the purpose of elevating the image forming speed and reducing the dimension of the apparatus, the photoreceptor is used under severer conditions in the repeated steps and the deterioration of the image quality by the mechanical action becomes conspicuous.
For example, in the case where a rubber blade is employed in the cleaning unit, a rubber material of a higher rubber hardness is employed for constituting the rubber blade in order to sufficiently clean the photoreceptor, thereby resulting in a higher contact pressure of the rubber blade to the photoreceptor and accelerating the abrasion thereof, whereby the photoreceptor shows a fluctuation in the potential or in the photosensitivity in the aforementioned repeated steps, leading to drawbacks of an abnormal image formation or a distorted color balance in the case of a color image formation.
In order to resolve such drawbacks, there are proposed technologies of forming a protective layer on the photosensitive layer of the photoreceptor or adding an inorganic filler in the photosensitive layer (see, for example, patent documents 2 to 7).
Patent document 1:JP 5-294005 APatent document 2:JP 1-205171 APatent document 3:JP 7-333881 APatent document 4:JP 8-15887 APatent document 5:JP 8-123053 APatent document 6:JP 8-146641 APatent document 7:JP 8-179542 A
However, the present inventors found that the image forming apparatus of the background art employing the surface emitting laser as the light source of the exposure device, including the image forming apparatus described in the foregoing patent document 1, has been associated with a drawback that a light amount on the photoreceptor becomes deficient because of following two reasons.
Firstly, there cannot be obtained a sufficient light emission amount per a light emitting point, because of a small volume of a cavity in the surface emitting laser itself. Secondly, in order to obtain a desired beam diameter, on the photoreceptor, while utilizing a surface emitting laser array having closely positioned light emitting points, there has to be provided an aperture in the scanning optical system, whereby the light amount is reduced. Also for attaining a higher resolution in the formed image with such aperture, it is necessary to utilize a smaller aperture, which however further reduce the light amount.
Also the present inventors found that, in the image forming apparatus described in the foregoing patent documents 1 to 7, the configuration of forming a protective layer on the photosensitive layer of the photoreceptor or adding an inorganic filler in the photosensitive layer improves an abrasion resistance, but, in the case where the image formation is continuously repeated over a prolonged period, there has resulted a potential increase in an exposed portion of the photoreceptor, thus leading to an image deterioration such as a decrease in the image density, whereby an image of a satisfactory image quality cannot be obtained. In particular, the present inventors found that the protective layer described in the patent document 7 has a high mechanical strength and improves the abrasion resistance but deteriorates the resolution of the image, showing thicker lines in a character image and being inadequate for attaining high image quality.
In this manner, in the image forming apparatus of the background art employing an exposure device with a scanning optical system utilizing a surface emitting laser array as the light source, it has been difficult to achieve downsizing of the apparatus and a higher image forming speed and, at the same time, to attain a higher quality (higher resolution) in the formed image and to maintain such image quality in a satisfactory state over a prolonged period.
For example, in order to achieve higher image quality, it is preferred a thinner photosensitive layer, which however shows a remarkable deterioration of the image quality resulting from the abrasion of the photoreceptor in the repeated use over a prolonged period. Particularly, in the image forming apparatus of the background art employing a surface emitting laser array as the light source, since the exposure light amount is relatively low as explained in the foregoing, even a slight abrasion of the photoreceptor results in a fluctuation in the potential and photosensitivity of the photoreceptor, particularly in a low sensitivity. Also in the case where the photoreceptor is made smaller in diameter in order to achieve downsizing of the image forming apparatus, the abrasion of the photoreceptor tends to be accelerated to limit the service life thereof, as it generally becomes necessary to employ a charger of contact type and the number of image forming cycles generally increases.